בחורף החימום במשרד עובד כל היום, אבל בבית אני מפעילה חימום רק בלילה.

ב־ is the preposition in / during. So בחורף means in winter or during the winter.

Hebrew often attaches short prepositions directly to the next word:

• בבית = at home / in the house
• בלילה = at night
• במשרד = in the office / in an office

חימום is a noun meaning heating.

In a sentence like this, it usually refers to the heating system or heating in general, not the act of warming something with your hands. It is similar to how English says the heating.

If you wanted to name a specific appliance, Hebrew would usually use a different word, such as a word for heater, radiator, and so on.

In the first clause, החימום means the heating: a specific heating system, namely the one in the office.

In the second clause, חימום is more general or less specifically marked. Hebrew often does this with mass or abstract nouns, even where English would still say the heating.

So the contrast is roughly:

• החימום במשרד = the heating in the office
• אני מפעילה חימום = I turn on heating / I turn on the heating

If you wanted the second clause to sound clearly definite too, אני מפעילה את החימום would also be very natural.

In Hebrew, machines, systems, and devices often work when they are on or operating. So החימום ... עובד means the heating is working / running / operating.

That is very natural Hebrew. It does not sound strange the way the heating works all day might sound in English.

Also, חימום is a masculine singular noun, so the form is עובד:

• masculine singular: עובד
• feminine singular: עובדת

Yes, sometimes you could, because פועל also means operating / functioning.

But in everyday speech, עובד is often the more common choice for things like appliances, systems, internet, air conditioning, and heating.

So:

• החימום עובד sounds very natural and colloquial.
• החימום פועל is possible too, but can sound a bit more formal or technical depending on context.

They describe two different things:

• עובד = is working / is running
  This describes the state of the heating.

• מפעילה = turn on / activate / operate
  This describes the action done by the speaker.

So the first clause says the office heating is already running all day, while the second says that at home the speaker personally turns on the heating only at night.

Because the speaker is female.

In the Hebrew present tense, the verb form agrees with the gender of the subject:

• אני מפעיל = I turn on / operate said by a male speaker
• אני מפעילה = I turn on / operate said by a female speaker

English does not show this difference, but Hebrew does.

Because את is used before a definite direct object.

Here the sentence has חימום without ה־, so it is not marked as definite. That is why there is no את.

Compare:

• אני מפעילה חימום = I turn on heating
• אני מפעילה את החימום = I turn on the heating

So the absence of את matches the absence of ה־.

Without vowel marks, forms like במשרד and בבית can be a little ambiguous in writing. Depending on context, they can mean either:

• in an office / in a house
• in the office / in the house

Here the context strongly suggests:

• במשרד = in the office / at the office
• בבית = at home

Especially בבית often means at home, not just physically inside a house.

Hebrew word order is fairly flexible, and putting בבית early gives it emphasis.

Here it highlights the contrast:

• במשרד ... all day
• אבל בבית ... only at night

So the sentence is structured to contrast the office with home. English can do the same thing:

• But at home, I only turn on the heating at night.

These are time expressions:

• כל היום = all day
• בלילה = at night
• רק = only

So:

• עובד כל היום = works all day
• מפעילה חימום רק בלילה = turn on heating only at night

The position of רק matters. Here it limits the time phrase בלילה, so the idea is only at night, not during the day.